The present invention relates to a manufacturing method of a thin-film magnetic head with a magnetoresistive effect (MR) multi-layered structure using exchange coupling magnetic bias, such as spin valve effect MR sensor, used for HDD (Hard Disk Drive) units.
Recently, thin-film magnetic heads with MR read sensors based on spin valve effect of giant MR (GMR) characteristics are proposed (U.S. Pat. Nos. 5,206,590 and 5,422,571) in order to satisfy the requirement for ever increasing data storage densities in today""s magnetic storage systems like hard disk drive units. The spin valve effect multi-layered structure includes first and second thin-film layers of a ferromagnetic material separated by a thin-film layer of non-magnetic and electrically conductive material, and an adjacent layer of anti-ferromagnetic material is formed in physical contact with the second ferromagnetic layer to provide exchange bias magnetic field by exchange coupling at the interface of the layers. The magnetization direction in the second ferromagnetic layer is constrained or maintained by the exchange coupling, hereinafter the second layer is called xe2x80x9cpinned layerxe2x80x9d. On the other hand the magnetization direction of the first ferromagnetic layer is free to rotate in response to an externally applied magnetic field, hereinafter the first layer is called xe2x80x9cfree layerxe2x80x9d. The direction of the magnetization in the free layer changes between parallel and anti-parallel against the direction of the magnetization in the pinned layer, and hence the magneto-resistance greatly changes and giant magneto-resistance characteristics are obtained.
The output characteristic of the spin valve MR read sensor depends upon the angular difference of magnetization between the free and pinned ferromagnetic layers. The direction of the magnetization of the free layer is free to rotate in accordance with an external magnetic field. That of the pinned layer is fixed to a specific direction (called as xe2x80x9cpinned directionxe2x80x9d) by the exchange coupling between this layer and adjacently formed anti-ferromagnetic layer.
In this kind of spin valve effect MR read sensor structure, the direction of the magnetization in the pinned layer may change in some cases by various reasons. If the direction of the magnetization changes, the angular difference between the pinned and free layers changes too and therefore the output characteristic also changes. Consequently stabilizing the direction of the magnetization in the pinned layer is very important.
In order to stabilize the direction of the magnetization by the strong exchange coupling between the pinned and anti-ferromagnetic layers, a process of temperature-annealing (pin anneal process) under an external magnetic field with a specific direction is implemented. The pin annealing is done as follows, first the temperature is elevated up to the Neel point under the magnetic field strength of 500 Oe to 3 k Oe, and held for about 30 minutes to 5 hours, and then cooled down to room temperature. By this pin anneal process, the exchange coupling is regulated at the interface of the pinned and anti-ferromagnetic layers toward the direction of the externally applied magnetic field.
However, the magnetoresistance characteristics may be changed under actual high temperature operation of a hard disk drive unit, even if the pin anneal processing is properly implemented. This degradation is caused by the high temperature stress during operation of the hard disk drive unit and by the magnetic field by a hard magnet layer used for giving a bias magnetic field to the free layer.
The detail of this degradation is as follows. The pinned direction of the magnetization in the pinned layer is different from that of the magnetic field (HHM) by the hard magnet. And hence the direction of the magnetization of the pinned layer which is contacted with the anti-ferromagnetic layer is slightly rotated toward the direction of HHM (hereinafter the direction of the magnetization of the pinned layer is expressed as xcex8p). In the anti-ferromagnetic material layer, the Neel point temperature differs from location to location inside the layer from macroscopic point of view, and it is distributed in a certain range of temperature. Even if the temperature is less than the xe2x80x9cbulkxe2x80x9d Neel point (average Neel point), there could be small area whose micro Neel point temperature is low and where the exchange coupling with the pinned layer disappears. When such spin valve effect MR read sensor is operated at a high temperature T, which is less than the blocking temperature at which the exchange couplings of all microscopic areas disappear, and then cooled down to usual room temperature, some microscopic area whose Neel temperatures are less than T is effectively annealed again and the direction of the magnetization is rotated to xcex8p. The total amount of the xcex8p rotated area by the temperature cycle determines the magnetic structure of the anti-ferromagnetic layer and the new direction of the magnetization of the pinned layer.
As stated in the above paragraph, usage of such spin valve MR read sensor at high temperature may cause a change of the pinned direction in the pinned layer, and the electrical output characteristics of the sensor are degraded in signal levels, and waveform symmetry.
Hereinafter, the degradation of the output characteristics of the sensor due to the rotation of the pinned direction will be described with reference to drawings.
The spin valve effect sensor operates by detecting change in its electrical resistance depending upon an angle between directions of magnetization in the pinned and free layers. The electrical resistance R is expressed by R=(1xe2x88x92cos xcex8)/2+xcex2, where xcex8 is the angle between directions of magnetization in the pinned and free layers and xcex2 is an electrical resistance (Rs) when the magnetization directions in the pinned and free layers are in parallel (xcex8=0 degree) as illustrated in FIG. 1a. When the magnetization directions in the pinned and free layers are in anti-parallel (xcex8=180 degrees) as illustrated in FIG. 1b, the electrical resistance becomes R=1+xcex2. Also, when the magnetization directions in the pinned and free layers are orthogonal (xcex8=90 degrees) as illustrated in FIG. 1c, the electrical resistance becomes R=xc2xd+xcex2.
As illustrated in FIG. 2, the spin valve effect sensor produces output voltage in response to the change in magnetization direction of the free layer caused by application of changing leakage magnetic field from the magnetic recording medium. Suppose that the direction of magnetization in the free layer rotates by +20 degrees (first magnetization state of the free layer) and by xe2x88x9220 degrees (second magnetization state of the free layer) due to the leakage magnetic field from the magnetic recording medium. If the pinned direction is normal, the resistance value across the sensor at the first magnetization state RF1 is RF1=(1xe2x88x92cos 70xc2x0)/2=0.329 and the resistance value across the sensor at the second magnetization state RF2 is RF2=(1xe2x88x92cos 110xc2x0)/2=0.671 as shown in FIG. 3a. Thus, the difference xcex94 R becomes as xcex94R=RF2xe2x88x92RF1=0.342. Whereas, if the pinned direction rotates by 20 degrees from the normal direction, the resistance value across the sensor at the first magnetization state RF1 is RF1=(1xe2x88x92cos 500)/2=0.178 and the resistance value across the sensor at the second magnetization state RF2 is RF2=(1xe2x88x92cos 90xc2x0)/2=0.500 as shown in FIG. 3b. Thus, the difference xcex94R becomes as xcex94R=RF2xe2x88x92RF1=0.322. Therefore, 20 degrees rotation of the pinned direction results degradation of 5.8% in the sensor output.
It will be understood from the above-description that thin-film magnetic heads with good output characteristics can be fabricated by stabilizing their pinned directions under high temperature environment. However, it is undesired to execute the pin anneal process of the magnetic heads for a long time in order to give stable pinned direction to them because total period of time for fabricating the thin-film magnetic heads increases.
It is therefore an object of the present invention to resolve the aforementioned problems, and to provide a manufacturing method of a thin-film magnetic head, whereby stable pinned direction at high temperature can be realized by a short time pin anneal process.
According to the present invention, a manufacturing method of a thin-film magnetic head with a MR multi-layered structure using exchange coupling magnetic bias, has a step of forming the MR multi-layered structure, and a step of providing the exchange coupling magnetic bias to the MR multi-layered structure by a temperature-annealing process. The temperature-annealing process includes a step of gradually decreasing the temperature of the multi-layered structure to a first predetermined temperature under application of magnetic field toward a predetermined direction.
Slow cooling pin anneal process for gradually decreasing the temperature of the multi-layered structure to a target temperature under application of magnetic field toward a predetermined direction (pinned direction) can give stronger exchange coupling within a short process period of time. Thus, a spin valve effect MR sensor with more stable direction of the magnetization of the pinned layer under high temperature atmosphere is realized. By stabilizing the direction of the magnetization of the pinned layer, the degradations of signal level and waveform symmetry of output waveforms under high temperature atmosphere can be greatly reduced. In addition, the manufacturing process can be shortened in time.
It is preferred that the gradually decreasing step is a step of decreasing the temperature of the MR multi-layered structure to the first predetermined temperature at a cooling rate within a range of 10xc2x0 C./hour to 50xc2x0 C./hour under application of magnetic field toward the predetermined direction. More preferably, the cooling rate is about 10xc2x0 C./hour.
It is preferred that the first predetermined temperature is a temperature higher than a room temperature. More preferably, the forming step includes a step of forming a MR multi-layered structure with a ferromagnetic layer and an anti-ferromagnetic layer using exchange coupling magnetic bias between the layers, and the first predetermined temperature is a temperature at which a reversed ratio of the anti-ferromagnetic layer becomes about 0.1.
The spin valve effect MR multi-layered structure may include first and second layers of a ferromagnetic material separated by a layer of non-magnetic electrically conductive material, and an adjacent layer of anti-ferromagnetic material formed in physical contact with the second ferromagnetic layer.
It is preferred that the temperature-annealing process is done at a dedicated heat treatment process under application of the magnetic field, independent from processes in wafer fabrication (from formation of the MR multi-layered structure on a wafer to just before dicing of the wafer into bars), that the temperature-annealing process is done at a part of another heat treatment process in wafer fabrication, or that the temperature-annealing process is done at a dedicated heat treatment process under application of the magnetic field, independent from processes in wafer fabrication and at a part of another heat treatment process in the wafer fabrication.
Preferably, at the independent dedicated process for temperature-annealing under application of magnetic field toward the predetermined direction, the heat treatment temperature is elevated to a second predetermined temperature (Neel temperature of the anti-ferromagnetic material of about 150 to 300xc2x0 C.) and sustained for a specified duration time, and then it is gradually cooled down.
Also preferably, the another heat treatment process is a resist curing process.
It is preferred that the resist curing process includes a process for elevating the heat treatment temperature to a second predetermined temperature and for sustaining it for a specified duration time under application of magnetic field toward a direction perpendicular to the predetermined direction.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.